Cast ductile iron pipe which is used for water mains generally utilizes joints which are designed to seal but allow flexibility in the joint. These joints do not keep the pipe from separating longitudinally. The pipe will separate any time there is a change in direction of the water flow, if the pressure is changed in the case of a valve or a fire hydrant being turned off, or if there is a pressure surge caused by a pump being turned on. Most of the pipe used for this purpose is plain on one end and belled with a gasket on the other end and commonly called "push-on". The plain male member fits into the gasket which is placed inside the bell of the next member.
Approximately fifty percent of pipe six inches in diameter and below is made of plastic, usually polyvinylchloride, with the balance being ductile iron. For pipe from 8 inches in diameter up to 16 inches in diameter, the percentage of ductile iron increases. At above 16 inches in diameter no plastic pipe used.
In order to keep this type of pipe joint from separating various devices have been developed which usually employ tie-rods which are mechanically secured to the male member and the female member to hold them together. These are expensive and difficult to install in the field. In some cases they will slip and there are many cities that will not allow their use because in some cases they have bent to the point that the gasket has leaked.
Another technique that is used to hold pipe joints together is to pour concrete around the joint. This is very expensive because of the cost of concrete and the labor involved. Also it is not possible readily to move or change the joint at a later date.